Mechanism for estimating and controlling di/dt-induced power supply voltage variations

ABSTRACT

A system for delivering power to a device in a specified voltage range is disclosed. The system includes a power delivery network, characterized by a response function, to deliver power to the device. A current computation unit stores values representing a sequence of current amplitudes drawn by the device on successive clock cycles, and provides them to a current to voltage computation unit. The current to voltage computation unit filters the current amplitudes according to coefficients derived from the response function to provide an estimate of the voltage seen by the device. Operation of the device is adjusted if the estimated voltage falls outside the specified range.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to power supplies and, in particular, tomechanisms for modeling the rate at which power supply voltages changein response to the current demands of integrated circuits.

2. Background Art

Over the past 25 years, power consumption by integrated circuits such asmicroprocessors, has grown from under one watt to over 100 watts. Thedramatic increase in power is a result of transistor scaling, which hasproduced many more transistors on a chip running at much higherfrequencies. Traditionally, voltage scaling has been used to reducepower to manageable levels. However, with supply voltages approachingone volt, further large reductions in voltage are not likely to provideadditional power reduction. The following discussion focuses onmicroprocessors, but it will be recognized that any integrated circuitsoperating at high frequencies and subject to varying work loads will besubject to the similar problems.

A microprocessor that consumes 100 W requires a power supply, voltageregulator, and power distribution network capable of supplying 100 W, aswell as a thermal solution (package, heat sinks, and fans) capable ofdissipating the resulting heat. Such components are costly and cannot beexpected to scale to higher power levels as transistor dimensionsshrink.

In addition to the absolute power levels, changes in power levels areproblematic. In particular, current fluctuations associated with rapidchanges in power level can cause the voltage seen by a device to moveoutside a specified range. A hypothetical 100 W microprocessor runningat 1.0V draws 100 A. To ensure proper circuit operation, a voltageregulator and power distribution network must maintain the supplyvoltage to within +/−5%. This means that no more than 100 mVpeak-to-peak ripple can be tolerated regardless of what themicroprocessor (or the software it is running) does. An ideal powerdistribution network will have sufficient capacitance, and small enoughinductance and resistance, to maintain the supply voltage to within 100mV even though the microprocessor's supply current may changedramatically within a few nanoseconds. This latter problem is referredto as the di/dt problem after the definition of inductance, V=L•di/dt. Vis the voltage across an inductor of value L when subject to a change incurrent di/dt. Practical power distribution networks do not currentlyprovide these ideal characteristics.

Mitigating the effects of di/dt on voltage levels becomes increasinglydifficult as power management techniques, like clock-gating, aredeployed in microprocessor designs. For example, a power-hungry unit,such as the floating-point execution unit, may be equipped with acircuit that turns on the clock when the unit is active and shuts offthe clock when the unit is inactive. Clock gating may be implementedwith extremely fine granularity—unit by unit and pipestage bypipestage—resulting in a large number of clock gating signals. Thistechnique minimizes power consumption in inactive units, but it resultsin large variations in overall power levels that depend on the softwarebeing run.

The present invention addresses these and other problems associated withpower delivery networks.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be understood with reference to the followingdrawings, in which like elements are indicated by like numbers. Thesedrawings are provided to illustrate selected embodiments of the presentinvention and are not intended to limit the scope of the invention.

FIG. 1 is a circuit diagram of an electrical model that represents apower distribution network suitable for use with a processor.

FIG. 2 represents a step response and an impulse response for the powerdistribution network of FIG. 1.

FIG. 3 is a flow chart representing one embodiment of a method inaccordance with the present invention for simulating voltage variationsin an integrated circuit.

FIG. 4 is a schematic representation of the method shown in FIG. 3

FIG. 5 represents the output of a power simulator for a processorexecuting a block of code over approximately 2000 clock cycles.

FIG. 6 represents the supply voltage of the processor simulated in FIG.5 over the same interval, as determined using a simulator in accordancewith the present invention.

FIG. 7 is a block diagram representing one embodiment of a computersystem employing a di/dt control mechanism.

FIG. 8A is a block diagram of one embodiment of a di/dt controller inaccordance with the present invention.

FIGS. 8B–8E are block diagrams of various embodiments of the current tovoltage computation unit of FIG. 8A.

FIG. 9 represents a simulation of the output power of the processor ofFIG. 7, operating under control of the current control unit of FIG. 8A.

FIG. 10 represents the variation in the supply voltage of the processorsimulated in FIG. 8A over the same interval.

FIG. 11 represents a simulation of the supply voltage variation versuspower as determined using a truncated convolution interval.

FIG. 12 represents a simulation of the supply voltage variation versusperformance determined using a truncated convolution interval.

DETAILED DESCRIPTION OF THE INVENTION

A mircroarchitectural simulator, such as SimpleScalar or SMTSIM,includes a model of a microprocessor's pipeline and control logic, aswell as an architectural simulator to simulate the flow of instructionsthrough the microprocessor's pipeline. The simulator determines how manyclocks are required to execute a given benchmark, and producesstatistics about events such as pipeline stalls, cache misses,mispredicted branches, and the like. Such simulators are well-known andextensively discussed in the literature. See for example, D. Burger andT. M. Austin, “The SimpleScalar Toolset, Version 2.0”, ComputerArchitecture News, Vole 25, No. 3 Jene 1997, pages 13–25, or D. M.Tullsen, “Simulation and Modeling of a Simultaneous MultithreadingProcessor”, 22^(nd) Annual Computer Measurement Group Conference,December 1996.

These and other simulators may be augmented to estimate powerconsumption, including the effects of clock-gating. For example, on eachclock interval, the simulator may determine which units and pipestagesare active. The simulator can compute the total power consumption duringthe clock by adding up the active power and idle power ofunits/pipestages (“blocks”) that are on and off, respectively. Theactive and idle powers for each block may be based on low-level circuitsimulations, estimates provided by circuit designers, measurements basedon actual circuits and the like.

In accordance with one embodiment of the present invention, the powersupply voltage delivered to a processor is modeled from the output of apower simulator, using an electrical model of the power distributionnetwork for the processor in combination with appropriate responsefunctions for the distribution network.

FIG. 1 shows an electrical model of a power distribution network 100 fora high performance microprocessor. The model includes decouplingcapacitors 110(a)–110(c) on the die, in the package, and in the voltageregulator, respectively. Also shown are parasitic inductances120(a)–120(d) and resistances 130(a)–130(d) associated with the package,socket, printed circuit board, and devices within the voltage regulator.The microprocessor is modeled as a variable current sink 140, and theremainder of the voltage regulator is modeled as an ideal voltage source150. Component values are chosen to be representative of those in theparticular system being modeled. The construction of such models isdiscussed in D. J. Herrell, B. Beker, “Modeling of Power DistributionSystems for High-Performance Microprocessors”, IEEE Transactions onAdvanced Packaging, Vo. 22, Issue 3, August 1999, pp. 240–248.

FIG. 2 represents the simulated response of power distribution network100 to application of a 25A current step (not shown). A sustainedcurrent increase of 25 A is applied to power distribution network 100,and the response (step response 210) of the supply voltage as seen bythe microprocessor, is modeled. Step response 210 may be determined, forexample, by running a Spice or comparable simulation on network 100.

The horizontal axis in FIG. 2 is divided into a sequence of timeintervals. The intervals may correspond, for example, to the clockcycles of a processor driven by power distribution network 100. For thedisclosed simulation, each interval corresponds to a 0.3 nanoseconds(“ns”) clock cycle, to represent the response of a processor clocked at3.3 GHz. Step response 210 dips and “rings” due to the inductance andcapacitance of power distribution network 100. It reaches a localminimum 220 approximately 30 clocks after initiation of the currentstep. Two exponentially decaying sinusoidal signals are discernable instep response 210 over the modeled interval. A high frequencycontribution is responsible for local minimum 220. This contribution maybe attributed to RLC resonances associated with on-chip components110(a), etc. A lower frequency contribution is responsible for a secondlocal minimum 250. This contribution may be attributed to RLC resonancesassociated with packaging and/or connector components, 110(b), 120(a),130(a), etc. A still lower frequency contribution, associated with powersupply components, may be apparent if the modeled interval is extended.

An impulse response 230 is also shown in FIG. 2. Here, impulse response230 represents the response of power distribution network 100 to asingle 25 A pulse of 0.3 ns duration. Impulse response 230 isproportional to the first derivative of step response 210. It may becomputed from step response 210 by taking the difference betweenadjacent samples of step response 210. Alternatively, it may be computeddirectly by modeling the response of network 100 to a 0.3 ns pulsehaving an amplitude of 25 A.

Step response 210 and impulse response 230 are examples of responsefunctions that may be used to characterize the behavior of a powerdelivery network to an electrical stimulus.

One embodiment of the present invention provides a mechanism fordetermining the voltage seen by a processor as a function of itsvariable activity over time. The activity may be represented as acurrent waveform or as discrete current pulses. These may be provided bya simulator, such as SimpleScalar or SMTSIM, that has been enhanced tomodel the effects of power consumption. As discussed in greater detailbelow, the activity may also be provided by an operating processor. Inthe latter case, a mechanism is provided for mitigating the effects ofdi/dt-induced voltage changes.

The disclosed mechanism relies in part on the observation that powerdistribution network 100 is, to rough approximation, a linear network.Linear systems are characterized by two properties: (1) scaling an inputto the system (current step or pulse amplitude) by a certain amountcauses the output to scale in proportion; and (2) applying a linearcombination of two inputs to the system produces the same output as doestaking the linear combination of the outputs produced by the system inresponse to each input, individually (superposition). The scalingproperty may be represented mathematically as f(c*x)=c*f(x) where c is aconstant. For example, doubling the amplitude of the input current stepin FIG. 2 to 50 A leads to an approximate doubling of the depth of localminimum 220. Superposition may be represented mathematically asf(x+y)=f(x)+f(y). In the example of FIG. 2, this means that response ofdistribution network 100 to first and second current impulses isreasonably approximated by summing the response of distribution network100 to the first current impulse and its response to the second currentimpulse, following an interval representing the delay betweenapplication of the first and second current impulses.

FIG. 3 is a flow chart representing one embodiment of a method 300 inaccordance with the present invention for estimating the voltageresponse of a power distribution network to an activity profile of anintegrated circuit. In accordance with the present invention, anactivity profile for the system is determined 310. The activity profilemay be a current waveform, a sequence of current pulses, or a comparableoutput of a simulator, as described above. If a processor is beingsimulated, the activity profile may be taken as the output of asimulator in response to a particular code sequence over a given timeperiod. For example, a series of current pulses (one pulse per clock)representing the current drawn by the active and inactive units of aprocessor on a series of clock cycles (“intervals”) may be calculated inthe manner described above. For an actual processor, the activityprofile may be provided by monitoring its current consumption on, e.g.,a clock by clock basis. A current waveform representing the activityprofile of the processor may be resolved into a series of currentpulses. In either case, the activity profile at selected intervals ischaracterized 320 by a size (amplitude of the current pulse) and offset(time of the interval relative to a reference time). For amicroprocessor, the selected interval may correspond to one or moreclock intervals. In the following discussion, the activity profile at aselected interval is referred to as a pulse.

The impulse response of a target power distribution network includingthe device, e.g. a microprocessor, is determined 330. For one embodimentof method 300, application of current step having a specified size andduration to the target network may be simulated, and an impulse responsedetermined from the simulated voltage response. Alternatively, theresponse of the power delivery network to a current impulse having aduration corresponding to the selected clock interval(s) may besimulated directly. If an actual system is available, it may be possibleto measure the impulse response directly.

The current pulses of the activity profile are then converted 340 to avoltage response of the power network, using the impulse response of thenetwork and a filter technique. Suitable filter techniques include, forexample, finite impulse response (FIR) filters, e.g. convolution, andinfinite impulse response (IIR) filters.

For one embodiment, an FIR filter scales and shifts each current pulse,according to its amplitude and offset, to provide a correspondingscaled, shifted impulse response. The scaled/shifted impulse responsesfor each pulse that generates a response in a given interval are thensummed to provide a voltage response of the power network in theinterval. The scaled/shifted responses of the network to the sequence ofpulses over the time period represent a convolution of the activitypulses with the response function of the network.

For another embodiment, coefficients are determined for an IIR filterthat reproduces the impulse response. The coefficients are applied tothe current pulses and, recursively, to earlier voltages determined fromthe current pulses, to indicate a voltage profile seen by the device.

FIG. 4 is a schematic representation of an FIR method described inconjunction with FIG. 3. In the illustrated example, an activity profile400 comprises current pulses 410, 420, 430 having amplitudes A₁, A₂, A₃,respectively. Current pulses 410, 420, 430 may represent, for example,the current drawn by a processor executing a code sequence on successiveclock cycles. Current pulses 410, 420, 430 are initiated at offset timesT₀, T₁, T₂, . . .

For purposes of illustration, a base impulse response 490 for aspecified current amplitude, e.g. 25 A, is represented as a sequence ofimpulse response amplitudes R₀–R₇ over 8 intervals (t₀–t₇). Impulseresponses 440, 450 and 460 represent base impulse response 490 (impulseresponse amplitudes R₀–R₈) 490 following scaling by amplitudes (A₁, A₂,A₃, . . . ) and shifting by offsets (T₀, T₁, T₂, . . . ) of pulses 410,420 and 430, respectively. Here, R_(ij) represents amplitude R_(j) ofimpulse response 490, scaled by current pulse amplitude A_(i). Assumingthe power delivery network characterized by base impulse response 490 islinear, R₁₀=c•A₁•R₀, R₃₂=c•A₃•R₂, and in general R_(ij)=c•A_(i)•R_(j)(Scaling). Shifting impulse responses 440, 450, 460 reflect thedifferent times at which their source current pulses 410, 420, 430,respectively, are initiated. In the disclosed example, T₀ is taken asthe reference time. Voltage waveform 470 represents the sum of impulseresponses 440, 450 and 460 in corresponding intervals, t₀, t₁, t₂, . . .(Superposition).

For example, voltage pulse V₀ represents an amplitude of impulseresponse 440 at time interval t₀ (R₁₀). Voltage response V₁ representsthe sum of impulse response amplitudes R₁₁ and R₂₀ in time interval t₁,and voltage response V₂ represents the sum of impulse responseamplitudes R₁₂, R₂₁, and R₃₀ in time interval t₂. Shifts indicated byoffsets T₁ and T₂ align impulse response amplitudes R₂₀–R₂₈, andR₃₀–R₃₈, of impulse responses 440 and 450, respectively, relative toamplitudes R₁₀–R₁₈ of impulse response 460.

In general, the voltage at an interval, t_(n), is the sum of the impulseresponse amplitudes for all current pulses that have non-zero amplitudeat t_(n). As indicated in FIG. 4, if the current pulse is initiated at atime, T_(k), the contribution of the current pulse to the voltage atinterval t_(n) is proportional to c•A_(k)•R_(n−k). Here, R_(n−k) is theamplitude of impulse response 490 in the interval n−k, where thedifference reflects the start time of the current pulse relative toreference time, T₀.

A current simulation for a microprocessor and a voltage response derivedfrom the current simulation in accordance with the present invention areshown in FIGS. 5 and 6, respectively. FIG. 5 represents a simulation ofthe power consumed by an Itanium™ processor of Intel® Corporation ofSanta Clara, Calif. The processor is simulated running a portion of anApache web server and gzip file compression program for 2000 clockcycles. Current curve 510 exhibits different phases of programexecution, each phase being characterized by efficiency, e.g.instructions per cycle (IPC), and current levels. In general, thoseclock cycles in which larger numbers of instructions execute in parallel(high IPC) are indicated by correspondingly higher current consumption,since more units of the processor are turned on. Conversely, clockcycles in which fewer instructions execute in parallel (lower IPC) havefewer units activated, and current consumption is correspondingly lower.Over the simulated interval, the processor's supply current varies from52 A to 72 A.

Phases 520, 530, 540 in which current levels oscillate rapidly andreproducibly between higher and lower power levels are associated withcode loops. Code loops execute the same instructions repeatedly withgreater and lesser levels of IPC. These phases are interleaved withphases 524, 534, 544, in which little loop activity is visible, andcurrent levels vary more randomly. The peak-to-peak current variationsin phases 524, 534, 544 are noticeably larger than those associated withloop phases 520, 530, 540. Beginning around clock cycle 1200, there isan extended interval in which the code section contains only a few shortloop segments. In this region, current consumption varies widely andunpredictably.

FIG. 6 represents a simulation in accordance with the present inventionof supply voltage variations 610 corresponding to the processor currentconsumption reflected in FIG. 5. Voltage curve 610 reflectsapproximately the events indicated in curve 510 of FIG. 5. For example,phases 620, 630, 640 correspond to loop phases 520, 530, 540,respectively, in which the peak-to-peak current varies regularly. Forthe modeled code sequence and processor, loop phases are characterizedby relatively high IPC and high current consumption. The supply voltagelevel drops in phases 620, 630, 640, reflecting increased currentconsumption by the processor. The voltage variations below clock cycle900 are relatively mild. However, above clock cycle 1152, where powerconsumption is more erratic, the voltage swings are significantlylarger. While these voltage variation still falls within the allowablerange for processors, the Apache/gzip workload does not represent aworse-case di/dt pattern.

The voltage variations evident in curve 610 can have seriousconsequences for processor operations. For example, the logic circuitsthat make up the processor require power supply voltages within acertain range to operate dependably. Voltage excursions outside thisrange may lead to computational errors or catastrophic circuit failuresin the processor. Simulations such as those indicated in FIG. 6 allowprocessor designers to study the impact on processor voltage of variouspower/performance tradeoffs. As discussed below in greater detail,real-time determinations of voltage profiles on actual systems may beused to limit the extent of voltage excursions.

A microprocessor's power delivery system is typically designed for theworst-case software that it is likely to run. The worst-case software isusually a program with extremely high IPC (for maximum powerconsumption), or a program that rapidly alternates between extremelyhigh IPC and extremely low IPC (for maximum di/dt). Such programs arereferred to as power viruses because they stress the power deliverysystem much more than normal application software. Clock-gating allows aprocessor that is designed for typical power consumption rather thanworst-case power consumption, to throttle its performance if it reachesa specified power threshold. However, clock-gating also magnifies thedi/dt effects on voltage.

The above-described method facilitates the development of on-die powercomputation and regulation hardware (“current control units”) forprocessors and other instruction-executing devices. In addition toproviding insights into how a processor's voltage may be impacted bycertain code sequences, these simulations allow variousdi/dt-mitigating-strategies to be tested at a pre-silicon stage. Thebenefit of insights that flow from the above-described simulations maybe appreciated by reference to a processor design that implements di/dtcontrol. One such embodiment is described below in greater detail.

FIG. 7 is a block diagram of one embodiment of a computer system 700 inwhich di/dt control is implemented. By way of example, computer system700 includes a processor 710, a main memory 740, a non-volatile memory750, various peripheral devices 760, system logic 770, and a powersupply 780. System logic 770 controls data transfers among processor710, main memory 740, non-volatile memory 750, and peripheral devices760. Power supply 780 provides power at a reference voltage for logicdevices in processor 710. Computer system 700 is provided to illustratevarious features of the present invention. The particular configurationshown is not necessary to implement the present invention. For example,system 700 may include multiple processors 710 or otherinstruction-executing devices, and the various components shown may becombined or eliminated in whole or in part. Systems such as personaldigital assistants (PDAs), smart phones, and the like may be expected toinclude devices and connections not shown in the disclosed embodiment.

Processor 710 includes multiple units 724, which form an instructionexecution pipeline 720. Instructions may be provided to processor 710from main memory 740, non-volatile memory 750 or other storage devices(not shown). The amount of current consumed by processor 710 isdetermined by the level of activity generated in various units 724 byinstructions (or lack thereof) in pipeline 720. A current control unit(CCU) 730 monitors current use by selected units 724 in response to theprocessed instructions and adjusts the activity level of processor 710accordingly. System 700 is shown including a single CCU 703. Otherembodiments may employ multiple CCUs 730, each of which controls di/dtlocally for one or more units 724.

As an instruction is staged down pipeline 720, it directs various units724 to perform one or more operations that, taken together, implementthe instruction. For example, a floating-point multiply-accumulateinstruction (FMAC) may cause the following operations to occur in theindicated resources: a floating point register file reads out threeoperands; an FMAC execution unit multiplies two of the operands and addsthe product to the third operation; an exception unit checks the productand sum for errors; and a retirement unit writes the result to thefloating point register file if no errors are detected. Depending on theparticular processor implementation, these resources or their componentsmay be grouped into one or more units 724, which are turned on and offas the instruction is staged down the pipeline.

Each unit 724 draws a certain amount of current when it is activated bythe instruction. For computer system 700, power supply 780 provides thecurrent while maintaining the reference voltage level within a specifiedrange. If activation or deactivation of one or more units 724 triggers asignificant change in the current demanded by processor 710, CCU 730responds to mitigate the change. One embodiment of CCU 730 takesadvantage of the behavior illustrated in the above-described simulationto mitigate the effects of di/dt on the reference voltage used byprocessor 710.

The activated/deactivated states of selected units 724′ may be monitoredto provide an estimate of the current consumed by processor 710 on agiven clock cycle. Selected units 724′ may be those that draw largecurrents when active, because these are likely to create sudden changesin di/dt when they are switched between their active and inactivestates. CCU 730 monitors the state of one or more selected units 724′ onsuccessive clock cycles to estimate a resulting current or voltage levelover a selected time period. The estimated level is compared with one ormore threshold values to determine whether the activity level of theprocessor should be adjusted. For example, if an estimated voltage levelis less than a first threshold, the flow of instructions throughpipeline 720 may be reduced. If the estimated voltage level is greaterthan a second threshold, a unit 724′ that might otherwise be deactivatedcan be left activated to reduce the rise in the voltage level.

One embodiment of CCU 730 estimates the voltage level seen by theprocessor by tracking the active/inactive state (activity state=1/0, forexample) of a selected unit 724′ on n-successive clock cycles (sampleperiod). The tracked states may be weighted, as discussed below, torepresent an activity waveform or profile for processor 710. Currentpulses that make up the profile may be analyzed using an impulseresponse suitable for the power delivery network of system 700 toprovide a profile of the voltage seen by the processor. If the voltagefalls outside a specified range, CCU 730 may alter the activity level ofprocessor 710 to offset the anticipated change.

For one embodiment of CCU 730, values representing the amplitudes ofeach of n-current pulses may be stored in a series of storage locations.A shift register capable of storing multi-bit amplitude values in eachentry may be employed for this purpose. As discussed below in greaterdetail, the number of entries is determined in part, by the accuracywith which the voltage seen by the processor is to be determined and thefilter algorithm employed. The entry in which an amplitude value residesrepresents the offset of the corresponding current pulse from the startof a sampling window.

Periodically, e.g. on each clock cycle, the voltage seen by theprocessor is estimated from the current amplitudes (A_(i)) stored in theentries of the shift register. For one embodiment of CCU 730, thecurrent amplitudes are staged through the shift register sequentiallyfrom entry 0 to entry n−1 (E₀ to E_(n−1)) on successive clock cycles.The shift register contents thus provide a snapshot of the device'sactivity level profile over an n-clock cycle window. At a given time,the most recent current pulse amplitude is in E₀, and it contributes tothe voltage seen by the processor an amount that is proportional to theamplitude value it stores ([E₀]) and to the network impulse response inthe first interval (R₀). Similarly, the next most recent pulse is inentry 1, and its contribution to the voltage is proportional to theamplitude value it stores ([E₁]) and the network impulse response in thesecond interval (R₁).

In general, the voltage response at a particular time may be estimatedas:

$V = {c{\sum\limits_{\imath = 0}^{n - 1}\;{\left\lbrack E_{\imath} \right\rbrack \cdot R_{i}}}}$That is, a weighed sum of the current amplitudes for the last n-clockcycle is calculated on each clock cycle, with weights provided by theimpulse response amplitude in corresponding intervals. The shiftregister shifts the current pulse amplitudes to provide the offsetrelative to the impulse response that was discussed in conjunction withFIG. 4. The shift register contents thus provide a sliding windowthrough which a running profile of the voltage levels seen by theprocessor can be estimated. In effect, this embodiment of CCU 730convolutes the measured activity profile of the processor with theimpulse response of its power delivery network to anticipate changes inthe voltage level seen by the processor. Convolution is an example of afinite impulse response (FIR) algorithm. As discussed below, currentcontrol circuits based on inifinite impulse response (IIR) algorithmsmay also be implemented.

For one embodiment of a system employing a CCU 730, a pipeline controlcircuit (FIG. 8A) adjusts pipeline activity to offset the anticipatedchange in voltage level if it falls outside an acceptable range. Theimpulse response may be modeled using a power supply network similar tothat of FIG. 2 with components designed to reflect the capacitive,inductive, and resistive properties of computer system 700.Alternatively, the impulse response of an actual system may be measured.

FIG. 8A is a block diagram representing one embodiment of CCU 730 andits interactions with selected units 724′ of pipeline 720. The disclosedembodiment of CCU 730 includes gate units 810(1)–810(n) (generically,gate unit 810), a monitor circuit 820, and a throttle circuit 830. Eachgate unit 810 controls power delivery to an associated unit 724′ inpipeline 720. For example, gate unit 810 may be a clock gating circuitthat couples or decouples a clock signal to unit 724′ according towhether or not the services of unit 724′ are used to implement aninstruction currently in the pipe stage(s) in which unit 724′ operates.Also shown in FIG. 8A is a pipeline control circuit 850 which indicatesto gate units 810 which units 724′ are active for the currentlyexecuting instructions.

For the disclosed embodiment of CCU 730, gate unit 810 provides a signalto monitor circuit 820 to indicate whether its associated unit 724′ isactive. The signal may be an activity state of unit 724′, which isasserted when unit 724′ is turned “on” and deasserted when unit 724′ isturned “off”. A typical processor may include 10–20 gate units 810 tocontrol power delivery to 10–20 units 724′. All units may be monitoredfor current changes, or subsets of the units may be monitored. Forexample, only those units, e.g. 724′, that consume large amounts ofcurrent may be monitored. As noted above, multiple CCUs 730 may beemployed to monitor multiple units or groups of units.

Monitor circuit 820 collects signals from gate units 810 over a samplingwindow and estimates a voltage seen by processor 710 based on thecollected signals. The disclosed embodiment of monitor circuit 820includes a current computation unit (ICU) 812, a current to voltagecomputation unit (IVCU) 814 and a threshold comparator unit (TCU) 816.As discussed below in greater detail, ICU 812 provides an estimate ofthe current consumed on successive clock cycles. IVCU 814 applies afilter algorithm to the estimated currents to generate an estimate ofthe voltage seen by the device, e.g. processor 710. TCU 816 compares theestimated voltage with one or more thresholds, and throttle circuit 830adjusts the activity level of processor 710, if indicated by thecomparison.

For the disclosed embodiment of CCU 730, ICU 812 includes weight units814(1)–814(n) (generically, weight units 814) and an adder 822826. Whenpresent, each weight unit 814 provides a first or second value to adder822 according to whether the activity state indicated by a gatingcircuit 810 for its associated unit 724′ is inactive or activate,respectively. The first value represents the current drawn by unit 724′if it is not activated, and the second value represents the currentdrawn by unit 724′ if it is activated. Adder 822 sums the valuesprovided by weight units 814 and outputs the sum as an estimate of thecurrent pulse drawn on each successive clock cycle by monitored units724′ of processor 710. Adder 822 and possibly weight units 814 may beeliminated if CCU 730 controls a single unit 724′. Also, if currentsdrawn by monitored units 724′ are roughly comparable, weight units 814may be eliminated.

IVCU 814 applies a filter algorithm to the current pulses provided byICU 812 to generate an estimate of the voltage seen by processor 710,and TCU 816 compares the estimated voltage with one or more thresholdvalues. If the estimated voltage exceeds the threshold value(s),throttle circuit 830 adjusts the activity level of processor 710 tocounteract the threshold crossing.

For one embodiment of monitor circuit 820, an estimated voltage valuethat falls below a lower voltage threshold indicates that processor 710may be drawing more current than the power delivery network can supplywhile remaining within the allowed voltage range. Under thesecircumstances, throttle circuit 830 may deactivate some units that wouldotherwise be activated. For example, throttle circuit 830 may cause afetch unit (not shown) in processor 710 to inject no-operations (NOPs)into pipeline 720. NOPs are essentially blank instructions that requireactivation of few if any units. Under these circumstances, performancedecreases since productive instructions are delayed while NOPs(unproductive instructions) run through pipeline 720. Power consumptionis also reduced since NOPs activate few, if any, units.

On the other hand, the estimated voltage may exceed an upper voltagethreshold when processor 710 is under-utilized and few units areactivated. For this case, throttle circuit 830 may activate, e.g.provide drive current to, units 724′ that would not otherwise be engagedby instructions currently in pipeline 720. For example, it may continueto deliver a clock signal to unused units 724′, causing their circuitsto charge and discharge despite the absence of correspondinginstructions. This increases the power dissipated by processor 710 overthat which is necessitated by the currently executing instructions.However, it has no direct impact on processor performance.

FIG. 8B is a block diagram of an embodiment of IVCU 814 suitable forimplementing the weighted sum voltage estimation algorithm (FIR)described above. An embodiment of TCU 816 is also shown. The disclosedembodiment of IVCU 814 includes an n-entry shift register 824 and anadder 826 receives its input signals through n-weight units850(0)–850(n−1) (generically, weight units 850). Shift register 824stores current amplitudes (A₀–A_(n−1)) for n-successive clock cycles inits entries. Weight units 850(0)–850(n−1) scale their respective inputs(A₀–A_(n−1)) from shift register 824 in proportion to the amplitudes ofcorresponding intervals of an impulse response for the power deliverysystem of interest. The disclosed embodiment of adder 826 implements theweighted sum of Eq. 1. That is, adder 826 effectively convolutes then-activity states with the impulse response of the power deliverynetwork including power supply 780 and processor 710.

The output of adder 826 represents the eestimated voltage response V_(E)of power supply network 784 to the current activity level of processor710. In FIG. 8B, V_(E) is provided to comparators 860(a) and 860(b) ofthreshold comparator 816 for comparison to upper and lower voltagethresholds, respectively. Comparator 860(b) determines if V_(E) dropsbelow the lower voltage threshold, and signals throttle circuit 830 toreduce current consumption if it does. Current consumption may bereduced by shutting off the clock directly, reducing instruction fetchor issue rates, or triggering some other current-conserving strategy.

Comparator 860(a) determines if V_(E) exceeds the upper voltagethreshold, and signals throttle circuit to increase current consumption(or at least maintain it at its current level). For example, throttlecircuit may increase current consumption by forcing on units that wouldotherwise be clock gated off, or it may maintain power by preventingclock gating from cutting off additional units.

Between the upper and lower thresholds, the device, e.g. processor 710,is allowed to run normally. The voltage computation, thresholdcomparison, and current adjustment (if necessary) may be performed everyclock or every m-clocks, depending on the control granularity required,the computational resources available and other factors. Real-timecontrol of power supply voltage is enabled, assuming the computationalresources required to estimate V_(E) at a suitable rate can beaccommodated on the processor die.

FIG. 8C is a block diagram of an embodiment of IVCU 814 that is suitablefor implementing an IIR or recursive filter. IIR filters employpreviously calculated filter outputs (e.g, earlier voltage estimatesfrom adder 826), in addition to the input signal (e.g. currents from ICU816) to determine a new filter output (e.g current voltage estimates).IIR filters are discussed in greater detail, for example, in Steven W.Smith, “The Scientists and Engineers Guide to Digital SignalProcessing”, California Technical Publishing ISBN 0-966176-3-3 (1997).In general, an IIR or recursive filter determines an output from alinear combination of input signals and previously calculated outputs,the latter providing the recursive element. An IIR filter function maybe represented as:

$V_{p} = {{\sum\limits_{i = 0}^{p}{A_{i} \cdot a_{i}}} + {\sum\limits_{i = 0}^{m}{V_{i} \cdot b_{i}}}}$Here, a₀–a_(p) represent the coefficients that apply to p currentamplitudes (A_(i)) and b₀–b_(m) represent the coefficients that apply tothe m prior voltage estimates (V_(i)). In general, if the last p currentvalues are tracked, p−1 estimated voltages are available for tracking,and m=p−1. The coefficients {a_(i)} and {b_(i)} may be determined bycurve fitting to the impulse response or step response (FIG. 2) of thepower delivery network.

The disclosed embodiment of IVCU 814′ includes a second shift register828 and weight units 854(1)–854(p−1) in addition to p-entry shiftregister 824, adder 826 and weight units 850(1)–850(p). Shift register824 stores a sequence of p current amplitudes and shift register 828stores a sequence of the preceding p−1 estimated voltages. The latterare feedback to adder 826 to provide the recursive input for IVCU 814.

IVCU 814′ is suitable for filtering current signals to provide anestimate of the voltage seen by processor 710 for the case in which stepfunction 210 or impulse response 230 is dominated by one decayingsinusoidal contribution. Multiple IVCUs 814 may be employed to generatevoltage estimates for cases in which there are multiple decayingsinusoidal contributions to the response of the power delivery network.Each sinusoidal contribution may be characterized by its own set ofcoefficients {a_(i)} and {b_(i)}. FIG. 8D shows an IVCU 814″ in whichthe results of k filters are combined in parallel to provide an estimateof the voltage seen by processor 710. FIG. 8E shows an IVCU 814′″ inwhich the results of k filters are combined in series to provide anestimate of the voltage seen by processor 710. Either configuration maybe use to combine IIR filter results for multiple resonance sources.

As indicated in FIG. 8A, CCU 730 includes a feedback loop, since theaction of forcing units 724′ on or off affects the computed supplycurrent and voltage. These in turn affect the subsequent behavior of CCU730. For IIR filters (FIG. 8C–8E), the recursive component provides anadditional feedback loop within IVCU 824.

Simulations of the effects of CCU 730 on current and power supplyvoltage are shown in FIGS. 9 and 10, respectively. Compared to FIGS. 5and 6, the di/dt control provided by CCU 730 introduces sharp one clockspikes to both the maximum and minimum current levels. The spikes are aresult of CCU 730 forcing the clock on or off in response to thecomputed voltage falling outside predetermined limits. Despite thespikes, the peak-to-peak variation in power supply voltage is less thanthe uncontrolled case because CCU 730, by its design, regulates thecurrent to avoid exciting RLC resonances in the power distributionnetwork.

The results of FIGS. 9 and 10 were simulated using an FIR algorithmapplied to the full range of impulse response 230 (FIG. 2), whichextends over more than 300 clock cycles. For CCU 730, this would amountto employing a shift register 824, weights 850 and adder 826 sizedaccordingly (n>300). Even with the circuit densities available intoday's high performance processors, it is not practical to implement amonitor circuit 820 capable of convoluting an impulse response withcurrent pulses for over 300 clock intervals in one clock cycle or evenin several clock cycles.

Simulation results indicate that a 300 clock cycle interval of FIG. 2 isnot necessary to provide results that are useful. For example, in FIG. 2the largest variation of impulse response 230, e.g. local maximum 240,occurs approximately 25 clock cycles following initiation of the currentpulse. This suggests that CCU 730 may convolute the current pulse andimpulse response over a shorter interval and still provide a reasonableapproximation to the voltage response of the power delivery network. Asdiscussed below in greater detail, monitor circuit 826 may be realizedwith n as small as 25 or 30.

One embodiment of the present invention allows the tradeoff betweenperformance and voltage stability to be simulated for differentconvolution intervals. Different performance levels may be tested byaltering the voltage threshold at which CCU 730 triggersdi/dt-mitigating operations. For example, if the lower voltage thresholdat which CCU 730 initiates activity-reducing operations, e.g. injectionof NOPs, is reduced, processor 710 continues operating at fullperformance over a wider voltage range but at a cost of exposing itscircuitry to greater voltage variations. Raising the lower voltagethreshold at which CCU 730 initiates activity-reducing operationsnarrows the range of voltages over which processor 710 is allowed tooperate at full performance. Overall, performance is reduced butprocessor 710 sees a more stable supply voltage.

FIG. 11 represents simulations of the peak-to-peak voltage variation ofa power delivery network for system 700 versus performance for threedifferent intervals. It is evident from FIG. 11 that there is littledifference in the results generated by the simulation for intervals of25 (curve 1110), 43 (curve 1120) and 350 (curve 1130) clock cycles. Ineach instance, peak-to-peak voltage variation may be limited toapproximately 1 mV with a performance reduction of less than 5%.

FIG. 12 represents simulations of the peak-to-peak voltage variation ofa power delivery network for system 700 versus dissipated power forthree different convolution intervals. The results are comparable forthe three intervals, indicating that as few as 25 intervals aresufficient to provide practical di/dt control.

The latency of CCU 730 can have significant impact on its efficacy incontrolling the voltage variations associated with di/dt swings. Forexample, it is apparent from FIG. 2 that the larger scale voltagevariations in step and impulse responses 210 and 230, respectively,occur within the first few intervals of a current change. Consequently,an effective CCU 730 is designed to respond with low latency, e.g. 1 or2 cycles. FIGS. 13 and 14 represent the dependence of peak-to-peakvoltage variation on performance and power for CCUs having latencies of1–4 clock cycles. It is apparent from these figures that much of thebenefit of di/dt control disappears for latencies >2 clock cycles.

For the disclosed embodiment of CCU 730, the main sources of latency areICU 812 and IVCU 814. As indicated above for FIR-based filters, reducingthe size of the monitored interval helps. In this regard, IIR-basedfilters are likely to provide better results for a given latency. Forexample, simulations have shown that reasonable estimates of the voltageseen by the device may be obtained using second order IIR filters. Asecond order filter employs only three current coefficients (a₀, a₁, a₂)and two voltage coefficients (b₀, b₁), which compares very favorablywith the 25 coefficients used by FIR filter 814′.

Other strategies may also be applied to reduce latency. For example, thecurrent consumption for blocks later in the execution pipeline can bepre-computed at an earlier stage. If necessary, two computations can beperformed at an early stage, and one selected later according to whetherthe relevant unit was active or inactive. In addition, for the disclosedembodiment of monitor circuit 820, only the first element of impulseresponse needs to be computed with one cycle latency. The second elementmay take two cycles, the third may take three cycles and so on.

Latency constraints also indicate that a single centralized CCU for,e.g. a processor, will not provide the same benefits as controllingdi/dt in a distributed/local manner. For example, a local CCU can bebuilt alongside the unit(s) whose activity it controls to respond fasterto current variations in its assigned unit(s). A centralized CCU mustawait data from units distributed across the chip. If local CCUs areassociated with different pipelines within a processor, somecommunication between pipelines will be necessary to ensure that dataexchanged between pipelines does not get lost. For example, queues maybe added between pipelines to buffer data from one pipeline that isdestined for another pipelined that is momentarily stalled by its CCU.

There has thus been provided a mechanism for simulating the voltage seenby a device in a power delivery network. The mechanism may be used tostudy the efficacy of various strategies for mitigating di/dt-inducedvariations in this voltage. For one embodiment of the invention, animpulse response is determined for the power delivery network, and anactivity profile for the device powered by the network is monitoredduring operation. The activity profile is then convoluted with theimpulse response to provide a profile of the voltages at the device.

This and other embodiments, which have been provided to illustratevarious features of the present invention, may be useful in modelingelectronic systems such as computers and other processor-based systemsthat employ clock gating or are subject to significant di/dt variationsfor other reasons. Persons skilled in the art and having the benefit ofthis disclosure will recognize variations and modifications of thedisclosed embodiments, which none the less fall within the spirit andscope of the appended claims.

1. A system to provide power in a specified voltage range to anintegrated circuit comprising: a power delivery network characterized bya response function; and a device to draw power from the network, thedevice including: a first shift register to store values representing asequence of current amplitudes drawn by the device on successive clockcycles; an adder having inputs weighted to reflect the response functionof the power delivery network, the adder to determine a weighted sum ofthe sequence of current amplitudes to estimate a voltage provided to thedevice; a voltage comparator to compare the estimated voltage to avoltage threshold; and a throttle unit to adjust operation of the deviceresponsive to the estimated voltage reaching the voltage threshold. 2.The system of claim 1, wherein the device is a processor that furthercomprises: an execution pipeline including one or more units to processinstructions; a fetch unit to deliver instructions to the executionpipeline; and a clock gating unit to control the delivery of power tothe one or more units, responsive to use of the unit by a currentlyexecuting instruction.
 3. The system of claim 2, wherein the throttleunit prevents the clock gating unit from gating off power to the one ormore units, responsive to the estimated voltage exceeding an upperthreshold voltage.
 4. The system of claim 2, wherein the throttle unitsignals the fetch unit to deliver instructions to the execution pipelineat a reduced rate, responsive to the estimated voltage falling below alower threshold voltage.
 5. The system of claim 1, wherein the responsefunction of the power delivery network is represented by n responsefunction amplitudes, and the weighted inputs of the adder comprises ninputs that are weighted proportionally to the n response functionamplitudes.
 6. The system of claim 1, wherein the response function ofthe power delivery network is represented by a set of recursioncoefficients, and the weighted inputs of the adder are weightedproportionally to the recursion coefficients.
 7. The system of claim 6,wherein the device further comprises a second shift register to track asequence of estimated voltages provided by the adder and to feedback thesequence of estimated voltages to selected inputs of the adder.
 8. Thesystem of claim 7, wherein the recursion coefficients include a set ofcurrent coefficients and a set of voltage coefficients.
 9. The system ofclaim 8, wherein adder inputs include first and second sets of inputscoupled to outputs of the first and second shift registers, the firstset of inputs being weighted proportionally to the current coefficientsand the second set of inputs being weighted proportionally to thevoltage coefficients.
 10. A system to provide power in a specifiedvoltage range to an integrated circuit comprising: a power deliverynetwork characterized by a response function; a processor core toexecute instructions, the processor core to draw power from the network,responsive to the instructions it executes; a monitor unit to estimate avoltage provided to the processor core, the monitor unit including: acurrent computation unit to track a sequence of current values drawn bythe processor core on successive clock cycles; and a current to voltagecomputation unit to filter the sequence of current values according tothe response function to provide an estimated voltage provided to theprocessor core; a threshold comparator to determine if the estimatedvoltage is within a specified range; and a throttle unit to adjustoperation of the processor core responsive to the estimated voltage notbeing within the specified range.
 11. The system of claim 10, whereinthe processor core includes multiple pipeline units that are selectivelyactivated, responsive to one or more of the instructions, each of themultiple pipeline units having an associated gate unit to provide acurrent signal to the pipeline unit if the pipeline unit is activated ona given clock cycle.
 12. The system of claim 11, wherein currentcomputation unit includes a shift register having a sequence of entriesto store current values for a sequence of clock cycles, the storedcurrent values to represent a sum of current signals provided to thepipeline units on each of the clock cycles of the sequence.
 13. Thesystem of claim 10, wherein the current computation unit includes anadder to estimate current amplitudes drawn by the processor core on asequence of m intervals and an m-entry shift register to store theestimated current amplitudes of m intervals.
 14. The system of claim 13,wherein the current to voltage computation unit includes an adder havingm inputs, each input being weighted according to the response functionof the power delivery network, the adder to estimate the voltageprovided to the processor core as a weighted sum of the m currentamplitudes provided by the m-entry shift register.
 15. The system ofclaim 14, wherein the response function is an impulse function for thepower delivery network and the m inputs of the adder are weightedproportionally to amplitudes of the impulse function at m intervals. 16.The system of claim 14, further comprising a p-entry shift register tostore a sequence of p estimated voltages provided by the adder, and tofeedback the p estimated voltages to inputs of the adder.
 17. The systemof claim 16, wherein the adder inputs coupled to the m-entry shiftregister are weighted proportionally to a first set of recursioncoefficients derived from the response function of the power deliverynetwork and the adder inputs coupled to the p-entry shift register areweighted proportionally to a second set of recursion coefficientsderived from the response function.
 18. An apparatus to estimate avoltage provided to a device in an integrated circuit comprising: ashift register having n entries to store values representing currentamplitudes on n successive intervals; n weight units, each weight unitto scale a current amplitude value from a corresponding entry of theshift register; an adder to sum the scaled current amplitudes from theweight units to provide an estimated voltage provided to the device. 19.The apparatus of claim 18, further comprising a second adder to sumcurrent amplitudes from one or more circuits and to provide the sum to afirst entry of the shift register, responsive to a clock signal.
 20. Theapparatus of claim 19, wherein each of the weight units stores a valuerepresenting a response function for a system in which the apparatus isto operate.
 21. The apparatus of claim 20, wherein the adder includes padditional inputs and the apparatus further comprises a second shiftregister having p entries to store a sequence of estimated voltages andto feedback the estimated voltages to the p additional inputs of theadder, the estimated voltages being provided by the adder.
 22. Theapparatus of claim 20, wherein the n and p inputs of the adder areweighted according to current and voltage recursion coefficients derivedfrom the response function.